Device and method for image compression and decompression

ABSTRACT

An image compression device includes a floating-point texture-loading unit, a shader, a RGB to YCrCb compressor, and a parameter adjusting unit. The floating-point texture-loading unit receives raw image data that includes N number of RGB pixels. The shader receives the raw image data and shades the N number of RGB pixels. The RGB to YCrCb compressor receives the shaded image data and calculates the brightness/chroma value of each of the RGB pixels, wherein the chroma value of the RGB pixel having the smallest brightness value among all the RGB pixels is selected as a common chroma value for all the RGB pixels. The parameter adjusting unit receives and adjusts N number of brightness values of the RGB pixels and the common chroma value and stores them in a form of compressed image data into a memory.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The invention relates to a device and method for image processing, andparticularly to a device and method for image compression anddecompression.

(b) Description of the Related Art

Typically, the wide range of luminance levels found in real scenes mayup to 100,000:1, which corresponds to information stored in high dynamicrange (HDR) images. However, the human eye may sense a range ofluminance levels only up to 10,000:1, which is narrower but is stillbroad enough to perceive tiny variations in luminance levels.

FIG. 1 shows a block diagram illustrating a conventional threedimensions (3D) graphic system 10. The 3D graphic system 10 includes agraphic processing device 11 and a memory 12. The graphic processingdevice 11 may be a 3D graphic card. The image data processed and storedby the 3D graphic system 10 are represented by fix-point numbers (i.e.,integers), and thus only a narrow dynamic range of luminance levelsbetween 0 and 1 can be obtained.

Taking an 8-bit fix-point number as an example, the value “00000000” isrepresented as “0.0f”, the value “11111111” is represented as “1.0f”,and thus its dynamic range is expressed as [0,1]. Since the fix-pointnumber representation may only express a narrow dynamic range, the imagerendered by the 3D graphic system 10 will be too dark or too bright asnot to accurately reflect luminance variations in real scenes.

In order to improve the narrow dynamic range provided by the 3D graphicsystem 10, a 3D graphic system 20 that utilizes a floating-point numberrepresentation is proposed to process and store image data. Referring toFIG. 2A, the 3D graphic system 20 includes a graphic processing device21 and a memory 22. The graphic processing device 21 may be a 3D graphiccard. The operation and architecture of the 3D graphic system 20 issimilar to the 3D graphic system 10, except the 3D graphic system 20utilizes the floating-point number representation to process and storeimage data. Hence, a dynamic range of luminance levels obtained by the3D graphic system 20 is expanded up to 100,000:1. Therefore, the 3Dgraphic system 20 can provide a high dynamic range (HDR) of luminancelevels. The reason of how the floating-point number representationachieves a high dynamic range of luminance levels is described below.

First, a typical floating-point number is expressed as:sign[exp].man   (1.1)where “sign” is the sign bit 1 or −1, “exp” is the exponent, and “man”is the mantissa. Further, a numeral expression of a floating-pointnumber can be written as:(−1)ˆsign*1.man*2ˆ(exp−15)   (1.2)

For example, a floating number s[5].10 indicates the exponent is 5 andthe mantissa is 10, and, substituting them into equation 1.2, thenumeral value of the floating number s[5].10 is found to have a maximumof 131008. Hence, when a float-point number representation of s[5].10 isused in the 3D graphic system 20, the maximum numeral value that the 3Dgraphic system 20 can designate is 131008. Hence, a dynamic range ofluminance levels obtained by the 3D graphic system 20 is expanded up to100,000:1 so as to accurately reflect luminance variations in a realworld.

FIG. 2B shows a block diagram illustrating the graphic processing device21. The graphic processing device 21 includes a floating-pointtexture-loading unit 21 a, a shader 21 b, and a parameter adjusting unit21 c. The parameter adjusting unit 21 c may be a rasterizer. Thefloating-point texture-loading unit 21 a reads raw image data Or havinga texture image format. Next, the shader 21 b receives and then shadesthe raw image data Or. The parameter adjusting unit 21 c receives andthen adjusts the shaded raw image data Or′ and outputs an adjusted imagedata At to the memory 22. The adjusted image data At to be stored in thememory 22 is in a form of a rendered target image format. The parameteradjusting unit 21 c adjusts the shaded raw image data Or′ by performingat least one operation of gamma correction, error correction, and colormixing. After stored the adjusted image data At, the memory 22 convertsthe adjusted image data At from a rendered target image format into atexture image format to improve image data access speed.

As described above, the 3D graphic system 20 utilizes a floating-pointnumber representation to process and store image data so as to expandthe dynamic range of the luminance levels. However, the occupied memoryspace required for the floating-point number representation is largerthan that for a fix-point number representation, and thus the dataamount handled by the graphic processing device 21 is considerably hugeto lower its processing speed and increase occupied memory space. Forexample, in a ARGB format (A is a color-mixing value), a pixel Prepresented as a 8-bits fix-point number requires a data amount of 32bits (=8*4) in graphic processing; however, the same pixel P representedas a 16-bits floating-point number requires a data amount of 64 bits(=16*4) in graphic processing. Thus, it is clearly seen thefloating-point number representation for a pixel will largely occupy thememory space.

Hence, though the 3D graphic system 20 may achieve a high dynamic rangeof luminance levels, the huge data amount it processes and stores may inturn lower its processing speed and increase occupied memory space toresult in an inferior performance.

BRIEF SUMMARY OF THE INVENTION

Hence, an object of the invention is to provide image compression anddecompression devices and methods that allows for reducing the dataamount and occupied memory space of image data representing asfloating-point numbers.

According to the invention, an image compression device includes afloating-point texture-loading unit, a shader, a RGB to YCrCbcompressor, and a parameter adjusting unit. The floating-pointtexture-loading unit receives raw image data that comprises N number ofRGB pixels (N is a positive integer) and are represented asfloating-numbers. The shader receives the raw image data and shades theN number of RGB pixels to generate shaded image data. The RGB to YCrCbcompressor receives the shaded image data and calculates thebrightness/chroma value of each of the RGB pixels, wherein the chromavalue of the RGB pixel having the smallest brightness value among allthe RGB pixels is selected as a common chroma value for all the RGBpixels. The parameter adjusting unit receives and adjusts N number ofbrightness values of the RGB pixels and the common chroma value andstores the N number of brightness values and the common chroma value ina form of compressed image data into a memory. The compressed image dataare also represented as floating-numbers.

The invention also provides an image decompression device that includesa floating-point texture-loading unit, a YcrCb to RGB decompressor, ashader, and a parameter adjusting unit. The image decompression devicedecompresses compressed image data that comprise N number of brightnessvalues (N is a positive integer) and a common chroma value for N numberof RGB pixels. The floating-point texture-loading unit receives thecompressed image data,and the YcrCb to RGB decompressor reads thecompressed image data and generating N number of RGB pixels convertedfrom N number of the brightness values and the common chroma value. Theshader shades the N number of RGB pixels, and the parameter adjustingunit receives and adjusts the N number of shaded RGB pixels and storesthe N number of shaded RGB pixels in a form of decompressed image datainto a memory. The compressed image data and the decompressed image dataare represented as floating-numbers.

According to the invention, since most chroma information not sensitiveto the human eye is discarded by means of the RGB to YcrCb compressor,and the mantissa of the common chroma value Cr′Cb′ is truncated tofurther decrease the data amount of the chroma information, the 3Dgraphic system of the invention may, under the condition that a highdynamic range of luminance levels still remains, have an enhance imageprocessing performance and a reduced occupied memory space.

Further, an image compression method performed by the image compressiondevice includes the steps of: providing raw image data including aplurality of RGB pixels represented as floating-numbers; calculating thebrightness/chroma value of each of the RGB pixels; selecting the chormavalue of the pixel having the smallest brightness value among all theRGB pixels as a common chroma value; and compressing the brightnessvalues of all the RGB pixels and the common chroma value to formcompressed image data represented as floating-numbers. Also, an imagedecompression method is provided for decompressing compressed image datathat represented as floating-numbers and comprise a plurality ofbrightness values and a common chroma value. The image decompressionmethod includes the steps of: reading the plurality of brightness valuesand the common chroma value, and converting the plurality of brightnessvalues and the common chroma value into a plurality of RGB pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating a conventional threedimensions (3D) graphic system.

FIG. 2A shows a block diagram illustrating another conventional 3Dgraphic system.

FIG. 2B shows a block diagram illustrating a conventional graphicprocessing device shown in FIG. 2A.

FIG. 3A shows a block diagram illustrating a 3D graphic system accordingto the invention.

FIG. 3B shows a block diagram illustrating the floating-point graphiccompressor shown in FIG. 3A.

FIG. 3C shows a block diagram illustrating a RGB to YCrCb compressoraccording to an embodiment of the invention.

FIG. 3D shows a block diagram illustrating another RGB to YCrCbcompressor according to an embodiment of the invention.

FIG. 3E shows a block diagram illustrating a floating-point graphicdecompressor according to an embodiment of the invention.

FIG. 3F shows a block diagram illustrating a YCrCb to RGB decompressoraccording to an embodiment of the invention.

FIG. 4 shows a block diagram illustrating a graphic processing deviceaccording to an embodiment of the invention.

FIG. 5 shows a block diagram illustrating a graphic processing deviceaccording to another embodiment of the invention.

FIG. 6 shows a flow chart illustrating an image compression method ofthe invention.

FIG. 7 shows a flow chart illustrating an image decompression method ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3A shows a block diagram illustrating a 3D graphic system accordingto the invention. Referring to FIG. 3A, the 3D graphic system 30includes a graphic processing device 31 and a memory 22. The graphicprocessing device 31 includes a floating-point graphic compressor 311and a floating-point graphic decompressor 312. A RGB/YCrCb conversionfor processing pixels represented as floating-point number is beingapplied to the graphic processing device 31.

Typically, the human eye may perceive tiny variations in brightness, butnot so in chroma. Thus, according to the invention, a common chromavalue Cr′Cb′ may replace all chroma values Cr₁Cb₁-Cr_(N)Cb_(N) (N is apositive integer) during a floating-point graphic procession toconsiderably reduce data amount.

FIG. 3B shows a block diagram illustrating the floating-point graphiccompressor 311 shown in FIG. 3A. The graphic compressor 311 includes afloating-point texture-loading unit 311 a, a shader 311 b, a RGB toYCrCb compressor 311 c, and a parameter adjusting unit 311 d. Theparameter adjusting unit 311 d may be a rasterizer.

The floating-point texture-loading unit 311 a reads raw image data Orfrom memory 22. The raw image data Or, represented as floating-pointnumbers, are in a texture image format and include N number of RGBpixels P1-P_(N). The shader 311 b receives the raw image data Or andgenerates shaded image data Or′ in which the N number of RGB pixelsP1-P_(N) are shaded. The RGB to YCrCb compressor 311 c receives theshaded image data Or′ and calculates the brightness/chroma value of eachof the RGB pixels P₁-P_(N), where the chroma value Cr_(Q)Cb_(Q) of thepixel P_(Q) (Q is a positive integer, 1≦Q≦N) that has a minimumbrightness among all the pixels P₁-P_(N) is served as a common chromavalue Cr′Cb′ for the N number of RGB pixels P₁-P_(N). The parameteradjusting unit 311 d receives N number of brightness values Y₁-Y_(N) andthe common chroma value Cr′Cb′, and then it adjusts these values byperforming at least one operation of gamma correction, error correction,and color mixing. The parameter adjusting unit 311 d stores the adjustedbrightness values Y₁-Y_(N) and common chroma value Cr′Cb′ in a form ofcompressed image data Cm into the memory 22. The compressed image dataCm are represented as floating-point numbers and in a rendered targetimage format. Further, in the memory 22, the compressed image data Cmare converted from a rendered target image format to a texture imageformat to accelerate reading of image data.

FIG. 3C shows a block diagram illustrating the RGB to YCrCb compressor311 c according to an embodiment of the invention. The RGB to YCrCbcompressor 311 c includes a RGB to YCrCb conversion unit 311 c 1 and achroma downsampler 311 c 2. The RGB to YCrCb conversion unit 311 c 1receives the shaded image data Or′ and calculates the brightness/chromavalue of each of the RGB pixels P₁-P_(N). Then, the chroma downsampler311 c 2 selects the chroma value Cr_(Q)Cb_(Q), which is the chroma valueof the pixel P_(Q) that has a minimum brightness among all the RGBpixels P₁-P_(N), as a common chroma value Cr′Cb′ for all RGB pixelsP₁-P_(N), and meanwhile discards other chroma values. Besides, a chromatruncating unit 311 c 3 may be additionally provided to aid compressioneffect. As shown in FIG. 3D, the chroma truncating unit 311 c 3truncates the mantissa of the common chroma value Cr′Cb′ and thenoutputs the truncated value.

FIG. 3E shows a block diagram illustrating the floating-point graphicdecompressor 312 according to an embodiment of the invention. Thefloating-point graphic decompressor 312 decompresses the compressedimage data Cm (represented as floating-point numbers and in a textureimage format) to generate decompressed image data Td (represented asfloating-point numbers and in a rendered target image format). Thefloating-point graphic decompressor 312 includes a floating-pointtexture-loading unit 311 a′, a YCrCb to RGB decompressor 312 c, a shader311 b′, and a parameter adjusting unit 311 d′. The parameter adjustingunit 311 d′ may be a rasterizer.

First, the floating-point texture-loading unit 311 a′ reads thecompression image data Cm stored in the memory 22. Then, the YCrCb toRGB decompressor 312 c reads the N number of brightness value Y₁-Y_(N)and the common chroma value Cr′Cb′ in the compression image data Cm andconverts them into N number of RGB pixels P₁-P_(N). The shader 311 b′receives and then shades the N number of RGB pixels P₁-P_(N). Next, theparameter adjusting unit 311 d′ receives and then adjusts each shadedpixel by performing at least one operation of gamma correction, errorcorrection, and color mixing, and the adjusted pixels are stored in aform of decompression image data Td into the memory 22. Similarly, thedecompression image data Td stored in the memory 22 are converted from arendered target image format to a texture image format according tospecific functions to improve image data access speed.

Note that the YCrCb to RGB decompressor 312 c can be implemented by aconventional converter such as an YcrCb to RGB conversion unit shown inFIG. 3F or other converter having fine conversion performance. Further,in this embodiment, the graphic processing unit 31 includes twofloating-point texture-loading units, two shaders, one RGB to YCrCbcompressor, one YCrCb to RGB decompressor, and two parameter adjustingunits. Alternatively, the two floating-point texture-loading units,shaders and parameter adjusting units may be respectively incorporatedinto one floating-point texture-loading unit, shader and parameteradjusting unit.

The following example illustrates the operations of downsampling andmantissa truncating achieved by the floating-point graphic compressor311 and corresponding decompression processes of the floating-pointgraphic decompressor 312 with reference to conventional chromaconversion and inverse chroma conversion equations.

First, the chroma conversion equation applied in the RGB to YcrCbconversion can be written as:Y=cr*R+cg*G+cb*B   (1.3)Cb=blue_diff*(B−Y)   (1.4)Cr=red_diff*(R−Y)   (1.5)where R,G, and B are input values of primary colors, Y is an outputvalue of brightness, Cr and Cb are output values of chroma, cr, cg, andcb are brightness parameters, blue_diff is a blue chroma parameter thatindicates blue color deviation, and red_diff is a red chroma parameterthat indicates red color deviation. The values of above parameters areassigned by a firmware and can be dynamically adjusted.

Next, the inverse chroma conversion equation applied in the YcrCb to RGBconversion can be written as:B=Y+(1/blue_diff)*Cb   (1.6)R=Y+(1/red_diff)*Cr   (1.7)G=Y+(−cr/(cg*red_diff))*Cr+(−cb/(cg*blue_diff))*Cb   (1.8)where the notation definitions have been previously mentioned and thusnot explain in detail.

Assume the RGB to YcrCb conversion unit 311 c 1 of the RGB to YcrCbcompressor 311 c receives four RGB pixels P₁, P₂, P₃, and P₄ whose RGBparameters are defined as:P ₁(R ₁ ,G ₁ ,B ₁)=(1000, 1200, 900)P ₂(R ₂ ,G ₂ ,B ₂)=(810, 900, 1090)P ₃(R ₃ ,G ₃ ,B ₃)=(900, 1000, 1200)P ₄(R ₄ ,G ₄ ,B ₄)=(1000, 1000, 1000)and the parameters set by the firmware are cr=0.3, cg=0.6, cb=0.1,blue_diff=1, and red_diff=1.

Thus, the chroma conversion equation T is obtained:Y=0.3R+0.6G+0.1BCb=1*(B−Y)Cr=1*(R−Y)

Thereafter, substitute the values of RGB parameters into the chromaconversion equation T to obtain respective brightness/chroma valuesY₁Cr₁Cb₁-Y₄Cr₄Cb₄ of the four RGB pixels P₁, P₂, P₃, and P₄:T(R ₁ ,G ₁ ,B ₁)=P ₁(Y ₁ ,Cr ₁ ,Cb ₁)=(1110, −110, −210)T(R ₂ ,G ₂ ,B ₂)=P ₂(Y ₂ ,Cr ₂ ,Cb ₂)=(892, −82, 198)T(R ₃ ,G ₃ ,B ₃)=P ₃(Y ₃ ,Cr ₃ ,Cb ₃)=(990, −90, 210)T(R ₄ ,G ₄ ,B ₄)=P ₄(Y ₄ ,Cr ₄ ,Cb ₄)=(1000, 0, 0)

Next, the chroma downsampler 311 c 2 receives the brightness/chromavalues Y₁Cr₁Cb₁-Y₄Cr₄Cb₄ and selects the chroma value of the pixel P2(Cr₂=−82 and Cb₂=198) as a common chroma value Cr′Cb′, on the reasonthat the pixel P2 has the smallest brightness value (Y₂.892) among thefour RGB pixels. Meanwhile, other chroma values Cr₁, Cb₁, Cr₃, Cb₃, Cr₄,Cb₄ are discarded. Then, the chroma truncating unit 311 c 3 receives thebrightness values Y₁-Y₄ and the common chroma value Cr′Cb′ and truncatesthe mantissa of the common chroma value Cr′Cb′(=−82.198) to obtain atruncated common chroma value Cr′Cb′(=−80,190). Finally, the parameteradjusting unit 311 d adjusts the brightness values Y₁-Y₄ and the commonchroma value Cr′Cb′ and stores these adjusted values in a form ofcompressed image data Cm into the memory 22.

Compared to the conventional design where all brightness and chromavalues Y₁, Y₂, Y₃, Y₄, Cr₁, Cr₂, Cr₃, Cr₄, Cb₁, Cb₂, Cb₃ and Cb₄ arerequired to be processed and stored together, the graphic processingdevice 31 only requires to process and store part brightness and chromavalues Y₁, Y₂, Y₃, Y₄, Cr′ and Cb′ in the memory 22 by means of thedownsampling and truncating operation of the invention. Hence, since thechroma values except the common chroma value are discarded by thedownsampling operation, the processing time as well as the occupiedmemory space is significantly reduced. Besides, the truncating operationperformed by the chroma truncating unit 311 c 3 may further reduce theoccupied memory space.

On the other hand, during decompression processes, the floating-pointtexture-loading unit 311 a′ of the floating-point graphic decompressor312 reads the compressed image data Cm that includes the brightness andchroma values Y₁, Y₂, Y₃, Y₄, Cr′ and Cb′ (=1110,892,990,1000,−80,190),and meanwhile the parameters of the inverse chroma conversion equationare set by the firmware as cr=0.3, cg=0.6, cb=0.1, blue_diff=1, andred_diff=1. Thus, the inverse chroma conversion equation T′ can bewritten as.B=Y+1*CbR=Y+1*CrG=Y+(−0.3/(0.6*1))*Cr+(−0.1/(0.6*1))*Cb

Next, substitute the values of the brightness and chroma values Y₁, Y₂,Y₃, Y₄, Cr′ and Cb′ into the inverse chroma conversion equation T′ toobtain respective values of the four RGB pixels P₁′, P₂′, P₃′, and P₄′:T′(Y ₁ ,Cr′,Cb′)=P ₁′(R ₁ ,G ₁ ,B ₁)=(1030,1118,1300)T′(Y ₂ ,Cr′,Cb′)=P ₂′(R ₂ ,G ₂ ,B ₂)=(812,900, 1082)T′(Y ₃ ,Cr′,Cb′)=P ₃′(R ₃ ,G ₃ ,B ₃)=(910,998, 1180)T′(Y ₄ ,Cr′,Cb′)=P ₄′(R ₄ ,G ₄ ,B ₄)=(920,1008,1190)

Thereafter, the parameter adjusting unit 311 d′ stores the RGB pixelsP₁′, P₂′, P₃′, and P₄′ in a form of decompressed image data Td into thememory 22. Thereby, the floating-point graphic decompressor 312 mayoutput an image almost the same as the original raw image data Oraccording to the brightness and chroma values Y₁, Y₂, Y₃, Y₄, Cr′ andCb′. Finally, the output image is provided for other image processingdevice or a display. According to the invention, since most chromainformation not sensitive to the human eye is discarded by means of thedownsampling operation, and the mantissa of the common chroma valueCr′Cb′ is truncated to further decrease the data amount of the chromainformation, the 3D graphic system of the invention may, under thecondition that a high dynamic range of luminance levels still remains,have an enhance image processing performance and a reduced occupiedmemory space.

Further, as shown in FIG. 4, the graphic processing device 31 mayadditionally includes a texture pre-loading unit 313. The texturepre-loading unit 313 is used to receive control signals that aretransmitted from a CPU 41 and passing through a north bridge 42. Whenthe texture pre-loading unit 313 receives the control signals, thememory 22 outputs image data assigned by the CPU 41 to the graphicprocessing device 31. Also, referring to FIG. 5, the graphic processingdevice 31 may further include a sequential processing unit 314 used tooutput image data to external display 51.

FIG. 6 shows a flow chart illustrating an image compression method ofthe invention, which includes the following steps:

Step S602: Start.

Step S604: Receive raw image data including multiple RGB pixels.

Step S606: Calculate the brightness/chroma value of each of the RGBpixels.

Step S608: Select the chorma value of the pixel having the smallestbrightness value among all the RGB pixels as a common chroma value.

Step S610: Truncate the mantissa of the common chroma value.

Step S612: Store all brightness values of the RGB pixels and the commonchroma value into a memory.

Step S614: End.

FIG. 7 shows a flow chart illustrating an image decompression methodcorresponding to the above image compression method, which includes thefollowing steps:

Step S702: Start.

Step S704: Read compressed image data from a memory. The image dataincludes multiple brightness values and a common chroma value.

Step S706: Generate multiple RGB pixels converted from the brightnessvalues in corporation with the common chroma value.

Step S708: End.

Note that the image compression and decompression device of theinvention are exemplified as processing 3D image data, but this is notlimited. The image compression and decompression device may also processone dimension (1D) and two dimensions (2D) image data.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art.

1. An image compression device, comprising: a floating-pointtexture-loading unit for receiving raw image data that comprises Nnumber of RGB pixels (N is a positive integer); a shader for receivingthe raw image data and shading the N number of RGB pixels to generateshaded image data; a RGB to YCrCb compressor for receiving the shadedimage data and calculating the brightness/chroma value of each of theRGB pixels, wherein the chroma value of the RGB pixel having thesmallest brightness value among all the RGB pixels is selected as acommon chroma value for all the RGB pixels; and a parameter adjustingunit for receiving and adjusting N number of brightness values of theRGB pixels and the common chroma value and storing the N number ofbrightness values and the common chroma value in a form of compressedimage data into a memory; wherein the raw image data and the compressedimage data are represented as floating-numbers.
 2. The image compressiondevice as claimed in claim 1, wherein the RGB to YCrCb compressortruncates the mantissa of the common chroma value.
 3. The imagecompression device as claimed in claim 1, wherein the parameteradjusting unit performs at least one operation of gamma correction,error correction, and color mixing on the N number of brightness valuesand the common chroma value.
 4. The image compression device as claimedin claim 1, wherein the RGB to YCrCb compressor comprises: a RGB toYCrCb conversion unit for receiving the shaded image data andcalculating the brightness/chroma value of each of the RGB pixels; and achroma downsampler for selecting the chroma value of the RGB pixelhaving the smallest brightness value among all the RGB pixels as thecommon chroma value.
 5. The image compression device as claimed in claim1, wherein the RGB to YCrCb compressor further comprises a chromatruncating unit for truncating the mantissa of the common chroma value.6. The image compression device as claimed in claim 1, wherein the rawimage data are in a texture image format and the compressed image dataare in a rendered target image format.
 7. The image compression deviceas claimed in claim 1,wherein the raw image data comprise one dimension(1D), two dimensions (2D), or three dimensions (3D) image data.
 8. Animage decompression device for decompressing compressed image data thatcomprise N number of brightness values (N is a positive integer) and acommon chroma value for N number of RGB pixels, the image decompressiondevice comprising: a floating-point texture-loading unit for receivingthe compressed image data; a YcrCb to RGB decompressor for reading thecompressed image data and generating N number of RGB pixels convertedfrom N number of the brightness values and the common chroma value; ashader for shading the N number of RGB pixels; a parameter adjustingunit for receiving and adjusting N number of shaded RGB pixels andstoring the N number of shaded RGB pixels in a form of decompressedimage data into a memory; wherein the compressed image data and thedecompressed image data are represented as floating-numbers.
 9. Theimage decompression device as claimed in claim 8, wherein the parameteradjusting unit performs at least one operation of gamma correction,error correction, and color mixing on the N number of shaded RGB pixels.10. The image decompression device as claimed in claim 8, wherein thecompressed image data are in a texture image format and the decompressedimage data are in a rendered target image format.
 11. The imagedecompression device as claimed in claim 8, wherein the compressed imagedata comprise one dimension (1D), two dimensions (2D), or threedimensions (3D) image data.
 12. An image compression method, comprisingthe steps of: providing raw image data including a plurality of RGBpixels represented as floating-numbers; calculating thebrightness/chroma value of each of the RGB pixels; selecting the chormavalue of the pixel having the smallest brightness value among all theRGB pixels as a common chroma value; and compressing the brightnessvalues of all the RGB pixels and the common chroma value to formcompressed image data represented as floating-numbers.
 13. The imagecompression method as claimed in claim 12, further comprising the stepsof truncating the mantissa of the common chroma value.
 14. The imagecompression method as claimed in claim 12, wherein the raw image dataare in a texture image format and read out from a memory.
 15. The imagecompression method as claimed in claim 12, wherein the compressed imagedata are in a rendered target image format and stored in a memory. 16.The image compression method as claimed in claim 12, wherein the rawimage data comprise one dimension (1D), two dimensions (2D), or threedimensions (3D) image data.
 17. An image decompression method fordecompressing compressed image data that represented as floating-numbersand comprise a plurality of brightness values and a common chroma value,the image decompression method comprising the steps of: reading theplurality of brightness values and the common chroma value; andconverting the plurality of brightness values and the common chromavalue into a plurality of RGB pixels.
 18. The image decompression methodas claimed in claim 17, wherein the plurality of RGB pixels are storedinto a memory in a form of decompressed image data represented asfloating-numbers.
 19. The image decompression method as claimed in claim17, wherein the compressed image data are in a texture image format andthe decompressed image data are in a rendered target image format. 20.The image decompression method as claimed in claim 17,wherein thecompressed image data comprise one dimension (1D), two dimensions (2D),or three dimensions (3D) image data.